1. Field of the Invention
The present invention relates to the field of seismic exploration. More particularly, the present invention relates to the time synchronization of data collected from multiple devices used in seismic exploration.
2. Art Background
In the art of seismic exploration energy generated by an artificial source is transmitted through the earth as seismic waves. At discontinuities within the earth the waves become partially reflected and consequently can be detected at the earth's surface with sensors such as geophones.
In land seismic surveys, hundreds to thousands of geophones may be deployed in a spatially diverse manner. Typically, several geophones are connected in a parallel-series combination on a single twisted pair of wires to form a single geophone group or channel. During the data collection process, the output from each channel is digitized and recorded for subsequent analysis.
The geophysical analysis of the recorded data requires knowledge of the precise locations of both the energy source creating the seismic waves and the channels or geophone groups detecting their reflections. If there are differences between the actual locations and the values for location used in data processing there could be problems with the analysis.
Typically, groups of geophones are placed at preplanned and presurveyed locations. The groups are usually connected to cables which in turn transport the collected data to recorders located at a central location. Alternately, some geophones systems transmit the collected data back via a radio link. As noted above the energy generated by the seismic energy source radiates in all directions.
In addition to pinpointing the locations of the geophones, it is important to synchronize the data received from the different numerous locations. The scientist analyzing the data is interested in the energy that gets reflected back towards the surface from the various stratigraphic layers and fault lines. The relative time of arrival at the geophones of these reflected signals of seismic sources is mapped over the prospect area. This mapping enables a map of the underlining earth's structure to be made. It is apparent that the geophones and associated recording devices must have data and time synchronization. Most systems specify timing accuracies in the order of tenths of microseconds.
Typically, systems use a tone issued by a single device, such as a central recording device or controller, to trigger time synchronization between the geophones/recording devices. For example, in cable-based systems a tone is sent down the cable to all geophones signaling the geophones to commence recording. When a single hop radio link is used, a tone is sent over the air to the geophones. Thus, using a tone based approach there is no need for explicit time specification.
However, there are situations where a tone based system to synchronize data collection is not adequate. One case is where the connections between the controller generating the tone and the geophones is via a packet radio link. This particularly is problematic when multiple hops are required to reach some of the geophones. A second situation is where neither cable or radio can be used and each geophone must record and operate in an autonomous manner. In this situation, the use of a clock with each geophone is required. However, time synchronization must be on the order of a few tenths of a microseconds; therefore a crystal clock would not suffice. Thus, it is desirable to provide a synchronization system that is not restricted by the above limitations.